Hi Paul,
I'd say because it is a not so straight-forward issue. One thing is, for example, to check and see if a web page complies with certain standard; to do this, services like that of the W3C Markup Validation are handy and useful, just like many other similar tools (e.g. Tidy). Another thing, however, is when a course creator has pasted data from Word, which usually comes with absurd amounts of formatting code, and has forgotten to use one of the available tools to clean-up the data; this case can require more than just using a particular tool. Last year, for example, I had to deal with a database in which teachers had created each and every quiz by pasting directly from Word; I don't even want to tell how much fun I had fixing this but I finally managed to bring a 230 MB sized Moodle question table to 13 MB.